Struct PivotExpander 

pub struct PivotExpander;

PIVOT Expander - Transforms PIVOT syntax into standard SQL with CASE expressions

This transformer converts SQL Server-style PIVOT operations into standard SQL using CASE expressions and GROUP BY clauses.

Transformation Example:

-- Input (PIVOT syntax):
SELECT * FROM food_eaten
PIVOT (MAX(AmountEaten) FOR FoodName IN ('Sammich', 'Pickle', 'Apple'))

-- Output (Standard SQL):
SELECT Date,
    MAX(CASE WHEN FoodName = 'Sammich' THEN AmountEaten ELSE NULL END) AS Sammich,
    MAX(CASE WHEN FoodName = 'Pickle' THEN AmountEaten ELSE NULL END) AS Pickle,
    MAX(CASE WHEN FoodName = 'Apple' THEN AmountEaten ELSE NULL END) AS Apple
FROM food_eaten
GROUP BY Date

The algorithm:

1. Detect PIVOT in FROM clause or JOINs
2. Extract PIVOT specification (aggregate function, pivot column, pivot values)
3. Generate CASE expression for each pivot value
4. Wrap each CASE in the aggregate function
5. Determine GROUP BY columns (all source columns except pivot_column and aggregate_column)
6. Build new SelectStatement with CASE expressions and GROUP BY

Implementations

impl PivotExpander

pub fn expand(statement: SelectStatement) -> Result<SelectStatement>

Transform a SELECT statement, expanding any PIVOT operations

pub fn expand_pivot( source: &TableSource, aggregate: &PivotAggregate, pivot_column: &str, pivot_values: &[String], alias: &Option<String>, ) -> Result<SelectStatement>

Expand a PIVOT operation into CASE expressions + GROUP BY

Trait Implementations

impl ASTTransformer for PivotExpander

fn name(&self) -> &str

Name of this transformer (for logging and debugging)

fn description(&self) -> &str

Description of what this transformer does

fn transform(&mut self, stmt: SelectStatement) -> Result<SelectStatement>

Transform the AST, returning the modified statement

fn enabled(&self) -> bool

Whether this transformer is enabled

fn begin(&mut self) -> Result<()>

Called before transformation starts (for initialization)

fn end(&mut self) -> Result<()>

Called after transformation completes (for cleanup)

impl Default for PivotExpander

fn default() -> Self

Returns the “default value” for a type.